Flashcards can often be flashed with ntrboot, which provides an alternate means of installing CFW. ntrboot is also useful for recovering units that are "bricked" to some degree.I started reading guides, and as far as I understand the only thing needed to get a CFW is a SD card reader. What's the purpose of all those flashcards sold on internet?
Otherwise, they are generally only useful for running DS games. TWiLight can also run DS games these days, but its compatibility is still not quite as good as what you can get with a flashcard – even if it's an old flashcard that has to be booted with NTR Launcher.
(And there also 3DS flashcards like the Gateway, but nobody advises using those anymore.)